1. Field of the Invention
The present invention relates to particulate matter detection elements used to an exhaust gas purifying system for an internal combustion engine of a motor vehicle, and are capable of detecting particulate matter contained in target detection gas such as exhaust gas emitted from the internal combustion engine.
2. Description of the Related Art
The present invention relates to particulate matter detection sensors mounted to an exhaust gas purifying system for an internal combustion engine of a motor vehicle and capable of detecting particulate matter contained in exhaust gas emitted from the internal combustion engine.
In general, a diesel engine, for example, mounted to a motor vehicle, is equipped with a diesel particulate filter (hereinafter, referred to as the “DPF”). Such a DPF captures particulate matter (hereinafter, referred to as the “PM” for short) as environmental pollution matter contained in exhaust gas emitted from the diesel engine. The PM contains soot and soluble organic fraction (SOF). The DPF is composed of a plurality of cells surrounded by partition walls having a plurality of pores. When the exhaust gas passes through the pores formed in the partition walls, the pores capture PM contained in the exhaust gas. The exhaust gas is thereby purified.
When a quantity of PM captured in the pores formed in the partition walls in the DPF is increased, the pores are clogged and a pressure loss of the DPF is thereby increased. In order to avoid this and to regenerate the capturing function of the DPF, it is necessary to periodically execute a process of regenerating the DPF.
In general, the regeneration cycle of the DPF is determined on the basis of detecting a quantity of PM captured in the DPF. It is therefore necessary to place a pressure sensor capable of detecting a difference between a pressure at a upstream side and a pressure at a downstream side of the DPF. The regeneration process heats the exhaust gas or executes a post injection in order to heat the exhaust gas, and introduces the heated exhaust gas into the inside of the DPF. This removes PM captured in the pores formed in the partition walls of the DPF.
On the other hand, there have been proposed various types of particulate matter detection sensors (hereinafter, referred to as the “PM detection sensor”) capable of directly detecting the presence of PM contained in exhaust gas. For example, such a PM sensor is placed at the downstream side of the DPF, and detects a quantity of PM contained in the exhaust gas passing through the DPF. An on-board diagnosis mounted to a motor vehicle monitors the output of the PM sensor in order to detect the working condition of the DPF, and occurrence of defects and damage of the DPF.
It has also been proposed to place such a PM sensor, instead of using a pressure difference sensor, at the upstream of the DPF, and to detect a quantity of exhaust gas introduced into the DPF. This can determine the optimum time of regenerating the DPF on the basis of the detected quantity of PM.
A conventional patent document 1, a Japanese patent laid open publication No. S59-197847, has disclosed a smoke sensor of an electrical resistance type as one example of the above PM sensor. The smoke sensor is comprised of an insulation substrate, a pair of conductive electrodes as a detection part, and a heating unit. The pair of conductive electrodes is formed on one surface of the insulation substrate, and the heating unit is formed in the inside or the bottom surface of the insulation substrate.
The smoke sensor detects the presence of smoke (particulate carbon) in exhaust gas on the basis of using electrical conductivity of the smoke. The smoke sensor detects the change of a resistance value between the conductive electrodes, which is changed according to the quantity of smoke accumulated on the area between the conductive electrodes.
The heating unit generates heat energy when receiving electric power. The heat energy increases a temperature of the PM detection part to a desired temperature (for example, a temperature within a range of 400° C. to 600° C.), and burns the smoke accumulated on the area between the conductive electrodes. This makes it possible to recover the detection capability of the smoke sensor.
In such a type of the PM detection sensor, the surface of the substrate other than the detection part is covered with an airtight insulation substrate in order to prevent a conductive path from being generated by PM accumulation on the surface of the part other than the detection part, and to prevent incorrect operation from thereby occurring. (For example, FIG. 1 shown in the patent document 1).
Further, such a type of the PM detection sensor has a large resistance value between detection electrodes until a predetermined quantity of PM is accumulated on the area between the detection electrodes on the detection part. In particular, there is known a dead time period (mass) during which the PM detection sensor outputs no detection signal.
In another (Japanese) conventional patent document 2, Kohyo (National publication of translated version) No. JP 2008-502892, discloses a conventional technique capable of changing a voltage supplied to the detection electrodes formed in a comb structure, and of increasing the supplied voltage at a detection initial period in order to increase an electric field intensity generated between the detection electrodes. This promotes a PM accumulation speed accumulated on the area between the detection electrodes, and decreases the dead time period. After completion of the dead time period, the conventional technique decreases the supply voltage in order to decrease the electric field intensity between the detection electrodes. This makes it possible to decrease the PM accumulating speed and to extend the period to start a process of regenerating the PM detection sensor.
By the way, when the PM accumulating speed is promoted by supplying a high voltage between a pair of the detection electrodes formed in a comb structure in which electrodes arranged opposite together, for example, as disclosed in the conventional patent document 2, a front part of each detection electrode has a high electric field intensity because of concentrating the electric field. On the other hand, the bottom part of each detection electrode has a low electric field intensity, which is connected to a corresponding detection electrode lead part and connected in a direction which is perpendicular to the corresponding detection electrode lead part formed along a longitudinal direction of the PM detection sensor.
When there is an area having a non-uniform distribution of electric field intensity (or non-uniform electric field intensity) on the detection part of the PM detection sensor, the PM accumulating speed is not constant, the quantity of PM accumulated on the detection part is fluctuated, In particular, as shown in the patent document 2, when the supplied voltage is increased in order to increase the quantity of PM accumulated on the detection part, the non-uniform distribution of electric field intensity occurs. In general, PM is more accumulated on the area having a high electric field intensity when compared with the area having a low electric field intensity. This increases a difference between the high electric field intensity and the low electric field intensity, and makes non-uniform distribution of PM accumulated on the detection electrodes. The more the degree of the non-uniform distribution is increased, the more the probability of generating incorrect output from the PM detection sensor is increased. This reduces the reliability of the PM detection sensor.